Acrolein frontier orbitals

Figure 1.3. Frontier orbital energies (eV) and confidents for acrolein and protonated acrolein. In the latter case the upper numbers refer to the situation where bond lengths and angles correspond to those of acrolein. The lower numbers are more suitable for a hydroxyallyl cation. The actual situation is assumed to be intermediate. The data are taken from ref. 104.

Fig. 8.2 Frontier-orbital coefficients and energies (eV) for acrolein and protonated acrolein [2 ...

Theoretical calculations have also permitted one to understand the simultaneous increase of reactivity and selectivity in Lewis acid catalyzed Diels-Alder reactions101-130. This has been traditionally interpreted by frontier orbital considerations through the destabilization of the dienophile s LUMO and the increase in the asymmetry of molecular orbital coefficients produced by the catalyst. Birney and Houk101 have correctly reproduced, at the RHF/3-21G level, the lowering of the energy barrier and the increase in the endo selectivity for the reaction between acrolein and butadiene catalyzed by BH3. They have shown that the catalytic effect leads to a more asynchronous mechanism, in which the transition state structure presents a large zwitterionic character. Similar results have been recently obtained, at several ab initio levels, for the reaction between sulfur dioxide and isoprene1. ... 

It has recently become clear113 114 that simple Hiickel theory, as well as some more elaborate techniques such as MINDO/2, are unreliable for use in conjunction with frontier orbital theory. For example, Hiickel molecular orbital coefficients suggest97 that acrolein (30) will dimerize to (31), but in fact the product is (32). SCF orbitals... 

The first calculations of the frontier orbitals for acrolein gave the HOMO coefficients on the C=C double bond of acrolein, with the a carbon having the larger coefficient. This failed to explain the regiochemistry, but only because the simple Hiickel theory that was used is notoriously weak in dealing with electron distribution in heteroatom-containing systems. Later calculations gave a better set of coefficients, as shown in Fig. 6.29. 

Fig. 4-67 Frontier orbital energies and coefficients for acrolein and protonated...

The first step is a straightforward ene reaction 160 with the LUMO of the aluminium complex of acrolein attracting the HOMO of the reactive alkene. The circles indicate the largest coefficient in each frontier orbital. The second step is an oxo-ene 161 with the oxygen atom of the aldehyde capturing the allylic proton. The stereochemistry of the product 159 suggests that the reactive conformation is 161. 

The Diels-Alder cycloaddition reaction of dihydropyran with acrolein was performed in the presence of various H-form zeolites such as H-Faujasites, H-p, H-Mordenites which differ both in their shape selective as well as their acidic properties. The activity of the different catalysts was determined and the reaction products were identified. High 3delds in cycloadduct were obtained over dealuminated HY (Si/Al=15) and Hp (Si/Al=25) compared to HM (Si/Al=10). These results were accounted for in terms of acidity, shape selectivity and microporosity vs mesoporosity properties. The activity and the regioselectivity were then discussed in terms of frontier orbital interactions on the basis of MNDO calculations for thermal and catalyzed reactions by complexing the diene and the dienophile with Bronsted and Lewis acidic sites. From these calculations, Bronsted acidic sites appeared to be more efficient than Lewis acidic sites to achieve Diels-Alder reactions. 

Figure 11.10. Frontier orbital control of regiochemistry for electron-rich dienes and acrolein.

FIGURE 3. Frontier molecular orbital energies (eV) and representations of the coefficients of acrolein, protonated acrolein and the acrolein-trifluoroborane complex101. Geometry optimizations were performed at the AMI1 °2,103 level of theory orbital energies and electronic distributions were determined at HF/3-21G104-107... 

Theoretical studies indicate that these transition structures are probably influenced by frontier molecular orbitals (in addition to steric effects), as indicated in Scheme 5.34c [182]. For the reaction of aminoethylene (a primary enamine) and acrolein, the enamine HOMO and the enone LUMO have the most attractive interactions when aligned in the chair configuration shown, which has the enone in an 5-cis conformation. Note that this orientation places the NH and the electrophile a-carbon in close proximity for proton transfer via the ene transition structure. 

Fig. 3 Frontier molecular orbitals calculated for formaldehyde and acrolein (Adapted from [75]...

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